This invention relates to polyolefin films having certain unique properties. Specifically, it relates to clear polyolefin films that exhibit an increased oxygen transmission rate as compared to conventional polypropylene films known to the art.
It is known that, for many applications, particularly packaging applications, polypropylene is less than totally satisfactory due to a relatively high permeability to oxygen. A great amount of effort has been expended in recent years to decrease the oxygen transmission characteristics of polypropylene. The best results to date have been obtained by metallization of the films.
There are, however, some applications in which a greater and controllable oxygen transmission rate is desired. This is particularly the case when fresh fruits, vegetables and flowers are to be packaged as these products continue to respire after they are packaged. Absence or insufficient levels of oxygen occurs as the oxygen originally present in the package is consumed, which leads to premature senescence and spoilage of the products. At the same time, respiration leads to a build-up of moisture in the package, which also can lead to spoilage of the product if the moisture cannot escape from the package. It is, thus, desirable to provide polypropylene packaging films in which the oxygen content can be replenished as necessary and from which the moisture can escape.
Conventional polypropylene films of a thickness required for most applications, whether monolayer or multilayer structures, have oxygen and moisture vapor permeability values that are not sufficient to allow the optimum oxygen and moisture levels to be maintained in a sealed package containing products of the type mentioned in the preceding paragraph.
One of the difficulties that typically arises in the design of films for use in the fresh cut produce packaging area is that in order to achieve progressively higher oxygen transmission rates (OTRs), the film stiffness or its tensile modulus normally decreases. For example, an oriented polypropylene (OPP) film has a higher modulus than a polyethylene (PE) film, but the OTR of the OPP film is significantly less than for the PE film of identical thickness. Due to the lower than desired OTR, OPP films have limited applicability in the packaging of various fresh-cut produce items.
It is known to prepare microporous films based on an opaque polymer mixture comprised of about 45% to 55% of a polypropylene homopolymer and 55% to 45% of a copolymer of propylene and ethylene containing about 2% to 5% ethylene by weight, which films have greater oxygen permeability than films made of polypropylene homopolymer alone. This polymer mixture is blended into a mixture comprised of about 40 to 60% of the polymer mixture and 60 to 40% of certain inorganic filler materials and is subjected to biaxial orientation. The filler material selected is one that causes voiding of the polymer matrix during the drawing operation. Exemplary of such voiding pigments are barium sulfate, calcium carbonate, silica, diatomaceous earth and titania, wherein calcium carbonate is the preferred filler material. U.S. Pat. No. 4,842,875 to Anderson teaches the use of such films in the preparation of controlled atmosphere containers for use with fresh vegetables, fruits and flowers.
Microporous films of the above described type function reasonably well having increased and, to a degree, controlled oxygen and moisture vapor permeability. However, the voiding pigments employed in the prior art are of a particle size large enough and are employed in concentrations great enough to result in formation of voids of such a size that the resultant films are almost totally opaque. Such opaque films are not suitable for many applications, such as in the packaging of fresh fruits and vegetables in which transparency is desired or required.
Polyethylene films also are known to have sufficiently great oxygen transmission qualities to permit produce items to continue respiring after being packaged without premature senescence and spoilage of such items. However, polyethylene films do not have a desired tensile modulus or optical properties for use in many packaging applications.
It is known in the prior art to mix polypropylene with other polymers to improve certain characteristics of plastic films.
U.S. Pat. No. 3,808,304 to Schirmer is directed to oriented blends of polypropylene and poly(1-butene) with improved heat shrinkablility.
U.S. Pat. No. 4,075,290 to Denzel et al. is directed to heat-sealable poly(1-butene) blends with an improved welding strength, the blends consisting essentially of 80-99% by weight of isotactic poly(1-butene) and copolymers of ethylene with propylene or 1-butene. Other blends consist essentially of 85-99% by weight of isotactic poly(1-butene) and isotactic polypropylene.
U.S. Pat. No. 4,311,571 to Mack describes a method of accelerating poly(1-butene) phase transformation to a stable type I polymorph in the presence of olefinic comonomers such as ethylene, polypropylene, 1-butene, etc., wherein poly(1-butene) is a predominant component.
U.S. Pat. No. 4,354,004 to Hughes et al. is directed to a film made from a composition consisting essentially of a blend of 90-98% of isotactic 1-butene-ethylene copolymer containing 0.5-10 wt % of ethylene, 2-9% of isotactic polypropylene homopolymer, and 0.02-1.5% of high density polyethylene, wherein the amount of polypropylene is critical.
U.S. Pat. No. 4,554,321 to Hwo et al. is directed to a film made from a composition consisting essentially of a blend of about 50-95% of isotactic 1-butene/ethylene copolymer containing 0.510 mole % of ethylene, from 50 to 5 wt % of stereoregular elastomeric polymer of 1-butene, and 2-9% of isotactic polypropylene.
U.S. Pat. No. 4,960,820 to Hwo is directed to a blend having 10% or less of a low molecular weight (150,000 or less), high melt index (100-1000), isotactic poly(1-butene) with 90% of polypropylene. In a preferred embodiment, the poly(1-butene) is a copolymer containing 1-30 wt % of ethylene or propylene.
Thermoplastic polypropylene blends with mixtures of copolymer elastomers of ethylene/butene and ethylene/octene are described by Srinivasan et al. in U.S. Pat. Nos. 5,763,534, 5,985,971, and 5,998,524. Polypropylene polymers disclosed by Srinivasan et al. have a melt flow rate (MFR) of about 10 to 80 dg/min. Ethylene/butene (EB) copolymers include ethylene in an amount of 5-95% and preferably about 50-90%. The resulting polymers have a MFR of at least 10 dg/min and can be used for molding or for mixing with paints to improve adherence.
U.S. Pat. No. 6,231,936 B to Kozimor et al. discloses a radiation tolerant polymeric blend containing polypropylene and a polymer produced by a single site catalysis (SSC-polymer), wherein the SSC-polymer comprises about 1-50% by weight of the blend. Examples 23-32 in Table 4B show blends of polypropylene having a MFR of 25 dg/min with SSC-produced ethylene-butene copolymers. However, the blends of polypropylene with ethylene-butene copolymers in this invention have a high haze (12.7-29.0%, Tables 4A-4B, column 14).
The patents discussed above do not disclose OTR values of resulting polymeric films by themselves or in relation to the film's tensile modulus and optical properties, e.g., haze and clarity.
U.S. Pat. No. 4,410,582 to Tsunashima et al. is directed to finger-tearable, multilayer structures usable as an adhesive tape substrate and in other applications. Although, this patent discloses the use of a nucleated polypropylene in the skin, the disclosed purpose of adding the nucleating agent is to improve the transparency of the film. There is no disclosure of providing a nucleated polypropylene having any required properties for improving the OTR of the film, let alone providing the high modulus properties required in the instant invention. In fact, there is no disclosure that controlling the tensile modulus properties of the polypropylene in the skin has any effect on the OTR. It is stated in column 8, lines 15-20 that the substrate may be usable as a packaging film if a paper or metal foil is bonded to one or both surfaces. However, employing a metallized or paper layer tends to reduce the OTR of the film and renders it opaque; thereby, making the film undesirable for packaging applications in which a high OTR and a low haze is required or desired.
European publication No. 0 243 965, describing an invention by Tsutomu, et al., discloses a multilayer packaging film for packaging fresh vegetables and fruits and including an antifogging additive in at least one surface layer that also has heat sealing properties. Also, the base layer of the anti-fogging multilayer film must contain, in addition to the homopolymer or copolymer of alpha-olefins, 10-70% of a polymer selected from the group consisting of vinyl acetate, acrylic acid and styrene (see page 8).
WO 95/26269 by Kuo discloses an oxygen-permeable multilayer film with an oxygen transmission rate of 500-50,000 cc/m2/24h STP, wherein an outer layer of the film consists of a homogeneous ethylene/α-olefin copolymer and has a density of 0.915 g/cc or less. The α-olefin is a C3-C20 α-olefin, preferably 1-butene. A second (core) layer is used to provide a desired tensile strength to the film and comprises a polypropylene/ethylene copolymer.
The following patents by the applicant herein relate to a search for a film with satisfactory OTR and tensile modulus values for use in packaging produce, flowers and the like. U.S. Pat. No. 6,232,402 relates to films based on a three component polyolefin blend and discloses a monolayer or a coextruded, multilayer film made from a blend of 75-92% of a high modulus, isotactic polypropylene (PP) homopolymer, 5-15% of a low density polyethylene (LDPE) (0.915-0.935 g/cc) and 310% of a heteropolymer containing polypropylene and at least one other 2 to 4 carbon a-olefin, wherein the heteropolymer is a copolymer or a terpolymer. In a preferred embodiment, films made from these blends exhibit an OTR of at least 200 cc/100 sq.in./day/atm, a tensile modulus of at least 195 kpsi and a haze value of no greater than 4%.
International publication WO 0012306A2 to DeMeuse and corresponding U.S. Pat. Nos. 6,485,817 and 6,514,625 disclose monolayer and multilayer polyolefin films. These references disclose a polyolefin film comprised of a blend consisting essentially of 10-75% by weight of a high modulus, isotactic polypropylene and 25-90% by weight of a heteropolymer consisting of at least two α-monoolefins. In a preferred embodiment, the heteropolymer is selected from the group consisting of a copolymer of ethylene and polypropylene containing about 4.5 to 6% ethylene by weight, a copolymer of polypropylene and 1-butene containing about 5 to 34% 1-butene by weight and a terpolymer of ethylene, polypropylene and 1-butene.
The range of uses for packaging of fresh produce that an oriented polypropylene (OPP) film can provide has recently been extended through the production of films based on blends of a high tacticity homopolymer polypropylene with propylene/ethylene copolymers. Specifically, a 75/25 blend of a propylene/ethylene copolymer containing 4% ethylene (Fina 8573 from ATOFINA (Deer Park, Tex.)) with a high tacticity polypropylene homopolymer (Amoco Accpro 9117 from BP Amoco (Alpharetta, Ga.)) has been shown to yield a film with an OTR at 1 mil thickness of about 190 cc/100 sq.in./day/atm and a machine direction tensile modulus of about 240,000 psi. The OTR value obtained is approximately twice that obtained with an OPP film based on homopolymer alone. In the film produced from the blend, the propylene/ethylene copolymer (Fina 8573) component provides the high OTR to the film, while the high tacticity homopolymer (Amoco Accpro 9117) component provides the required or desired stiffness.
However, the observed trend is that if the amount of the propylene/ethylene copolymer (Fina 8573) is increased in the blend to increase the OTR, the tensile modulus of the resulting film is reduced. Similarly, if the amount of the propylene/ethylene copolymer (Fina 8573) is decreased, the tensile modulus of the resulting film increases, but with a decrease in the film's OTR. If copolymers, which contain either more or less than 4% ethylene are used in blends with polypropylene homopolymer, the same correlation of increase/decrease between OTR and tensile modulus values is observed.
Thus, the prior art films exhibit a problem in that, although the films made from blends of propylene/ethylene copolymers with high tacticity homopolymer polypropylene have OTR values approximately twice that of a conventional OPP film, they do so at the expense of the films' tensile modulus and/or haze. Such negative correlation between the film's OTR and tensile modulus is likely to create complications in production and handling of the final film. Therefore, a need exists for a film having higher OTR, low haze (below 4%) and improved tensile modulus values. Also, there is a need to produce a film having higher OTR and low haze with similar tensile modulus values as the film based on the propylene/ethylene copolymer blends with the high tacticity homopolymer polypropylene. Further, there is a need for a film having similar OTR values as the film based on the blends of the propylene/ethylene copolymer with the high tacticity homopolymer polypropylene, but with higher tensile modulus and low haze.
The current invention provides a unique and desired balance among OTR, tensile modulus, and optical properties in packaging films. As will be discussed in greater details hereafter, the inventor obtained unexpected results by blending a high modulus polypropylene with 1-butene/ethylene copolymers.
It is an object of this invention to provide polyolefin compositions and films based on blends of polypropylene and copolymers of 1-butene with ethylene that have a desired balance of properties required to be useful in modified atmosphere packaging applications for packaging fresh vegetables, fruits, flowers and the like.
All references cited anywhere in this application are incorporated herein by reference in their entireties.